1. Field of the Invention
This invention relates generally to an apparatus for recording and/or reproducing a PCM (pulse-code modulated) audio signal by a rotary head, and more particularly is directed to an apparatus for recording an index signal, such as, a signal indicative of a starting point of a recorded event or program, in an oblique track in a so-called after-recording mode, or for erasing such index signal.
2. Description of the Prior Art
In accordance with the international standardization thereof, an 8 mm video tape recorder can be used in a normal recording mode in which an audio signal is frequency modulated prior to being mixed with a color video signal and then recorded in such a way that the audio signal can be frequency separated from the color video signal upon playback. Further, an optional recording mode can be employed in which the audio signal is pulse-code modulated and recorded in a so-called overscan area which is separate from the main area of each oblique track in which the color video signal is recorded. This latter mode is hereafter referred to as the normal PCM mode.
As is shown on FIG. 1, a rotary head assembly of a typical 8 mm video tape recorder includes recording and reproducing rotary magnetic heads HA and HB. These rotary magnetic heads HA and HB have gaps with different azimuth angles and are mounted on a rotary drum 1 with an angular spacing of 180.degree. therebetween. Rotary heads HA and HB are rotated at a rotational speed equal to the standard frame frequency (30 Hz) in the direction indicated by arrow 3H and protrude slightly from the peripheral surface of the rotary drum 1. A magnetic tape 2 is wrapped around the peripheral surface of rotary drum 1 over an angular extent of 221.degree. and is suitably transported at a constant speed in the direction indicated by an arrow 3T.
Accordingly, tracks 4A and 4B, each having a length corresponding to an angular extent of 221.degree. on the surface of drum 1, are alternately formed on tape 2 by rotary heads HA and HB, respectively, as shown in FIG. 2. In a so-called overscan area AP of each track having an angular extent of 36.degree. measured from a point at which the rotary head HA or HB starts scanning the track 4A or 4B, there is recorded an audio signal corresponding to one field period of the video signal and which has been pulse-code modulated and time compressed. On the succeeding main area AV of each track having an angular extent of 180.degree., there are recorded a color video signal of one field period, an FM (frequency modulated) audio signal of one field and tracking pilot signals, respectively. The remaining area of each track which has an angular extent of 5.degree. is assigned as a spare area in which the head is disengaged from the tape.
Further, the PCM audio signal can be recorded and reproduced by the 8 mm video tape recorder in a so-called "multi-PCM" or "audio-use" mode in which, as disclosed in detail in U.S. Pat. No. 4,542,419, issued on Sept. 17, 1985 and having an assignee in common herewith, the main recording area AV of each track is also used for recording the PCM audio signal. In such audio-use or multi-PCM mode, the 8 mm video tape recorder becomes exclusively a PCM audio signal recording and/or reproducing apparatus in which PCM audio signals can be recorded in multiple channels. More specifically, as shown on FIG. 3, in the audio-use or multi-PCM mode, the area AV of each track corresponding to the angular range of 180.degree. is divided equally by 5 and the PCM audio signal is recorded in a selected one of these divided portions during scanning of each track. Therefore, the whole area of each track shown on FIG. 2, including the area AP and the area AV, is divided into 6 track areas AP1 to AP6, each corresponding to an angular range or extent of 36.degree., and first to sixth track channels are formed by the same numbered segments or track areas AP1 to AP6 of the respective skewed tracks 4A,4B,4A, 4B, etc., shown on FIG. 3, whereby a suitably time compressed PCM audio signal can be recorded in any desired channel. For example, the PCM audio signal may be recorded first in the segments or track areas AP1 of the successive tracks 4A,4B, from one end of the tape to the other. Thereafter, the PCM audio signal may be further recorded in segments or track areas AP2 of the successive tracks from one end of the tape to the other. Accordingly, PCM audio signals can be recorded in and reproduced from each of the six channels with the result that, in the audio-use or multi-PCM mode shown on FIG. 3, the tape 2 has a recording time or capacity six times as long as when a PCM audio signal is recorded only in the overscan area AP of each track, as on FIG. 2.
In the case of the multi-PCM mode, if recording and/or reproducing is effected in or from each segment track area, the PCM signal processing circuit used therefor may be the signal processing circuit provided for processing the PCM audio signal recorded in the single channel constituted by the track areas AP in the prior art 8 mm video tape recorder.
The track format of the above mentioned 8 mm video tape recorder in its normal PCM mode will now be described more fully with reference to FIG. 4, in which contact of the rotary head with the tape 2, that is, the starting point of the track, begins at the right-hand side where there is provided a tracing starting area 11 corresponding, in extent, to a 5.degree. rotation angle of a rotary head. At the rear or trailing portion 12 of tracing starting area 11, a period having an angular range or extent of 2.06.degree. and corresponding to 3H of the video signal, where H is the horizontal period, is assigned as a preamble area which will become a clock run-in area synchronized with the succeeding PCM data. Following the preamble or clock run-in area 12, there is provided a PCM data recording area 13 having an angular extent of 26.32.degree. and in which a time compressed PCM audio signal is recorded. A postamble area 14 also having an angular extent of 2.06.degree. (3H) follows the PCM data recording area 13 so as to be used as a back margin area to cope with the displacement of the recording position when the recording is carried out in the so-called after-recording mode. A next area 15 having an angular extent of 2.62.degree. is assigned as a guard band area for separating the video signal area 16 which follows from the PCM data area 13. The video recording area 16 has an angular range or extent of 180.degree. as earlier noted for receiving the recorded video signal of one field period. Next to the area 16, there is provided a head disengaging area 17 having an angular extent of 5.degree. and in which the rotary head is disengaged or separated freely from the magnetic tape.
A track format used in the multi-PCM mode will now be described with reference to FIG. 5 in which it is shown that the format for each segment track area accommodating the PCM audio signal is exactly the same as the format for the PCM audio area AP in the normal 8 mm video tape recorder. Thus, each segment track area in FIG. 5 is formed of a tracing starting area 21, a preamble area 22, a PCM data area 23, a postamble area 24 and a guard band area 25. The same format is assigned to each of the segment track areas AP1 to AP6 on FIG. 3.
Generally, the PCM data is recorded on the tape with the binary code data having a logic level "1" or "0" being modulated to signals of respective frequencies. In the 8 mm video tape recorder, for example, the data having the logic level "1" is modulated to, or represented by a signal having a frequency of 5.8 MHz which is recorded, while the data having the logic level "0" is modulated to, or represented by a signal having a frequency 2.9 MHz which is recorded. In the prior art, for example, U.S. Pat. No. 4,551,771, issued Nov. 5, 1985, only the data having the logic level "1", that is, the signal having the frequency 5.8 MHz, is recorded in each preamble area 12 or 22 and in each postamble area 14 or 24.
A so-called program search method for locating a starting point of an event or program recorded by the 8 mm video tape recorder in the normal PCM mode and the multi-PCM mode has been proposed by a person or persons having a duty of assignment to the assignee of the present application and is described in U.S. patent application Ser. No. 838,626, filed on Mar. 11, 1986. In accordance with such method, an index signal is recorded in, for example, the postamble area 14 or 24 of each track format described above, and this index signal is used to access the starting point of the respective recorded event on the tape or program. Moreover, the index signal can be recorded and/or reproduced by the rotary heads used for the color video and/or PCM audio signals so that stationary recording and reproducing heads are not required for the index signal. Further, since the index signal is recorded in an area 14 or 24 separated from the track areas 13 or 23 and 16 for recording of the PCM data and the video signal, respectively, it is possible to insert or erase the index signal in an after-recording mode.
Although it is described above that the index signal is recorded in the postamble area, strictly speaking, the index signal is recorded in an area spanning the postamble area 14 and the guard band area 15 and has a code length of about 3H, where H is the horizontal interval, as indicated at IDX in FIG. 4. In any case, in order to record the index signal and/or erase the same, it is necessary to form an area specifying signal which specifies this postamble area as an area for an index signal, whereupon, the recording and/or erasing of the index signal must be carried out within the area specified by the area specifying signal.
Such index area specifying signal can be generated from a switching signal RFSW (FIG. 6C) which changes-over connections to the two rotary heads HA and HB. In other words, the switching pulse RFSW may be derived from a pulse signal indicating the absolute rotary phases of the rotary heads HA and HB and which is provided by a pulse generator associated with the usual rotary drum motor. The leading and trailing edges of the switching pulse RFSW, which become the times for switching from one to the other of the rotary heads HA and HB, are located at times within the guard band area 15 on the track format. FIGS. 6A to 6D illustrate the relationships among the record tracks HA and HB formed by the two rotary heads HA and HB, switching signal RFSW, and a PCM area signal SA which indicates each PCM segment track area AP. In the multi-PCM mode, both the switching signal RFSW and the PCM area signal SA are phase-shifted by a suitable multiple of 36.degree. relative to the absolute rotary phase (reference phase for the drum phase servo) with the phase relationship shown in FIGS. 6C and 6D, being maintained between signals RFSW and SA. Accordingly, if, for example, a delaying monostable multivibrator is triggered at the leading and trailing edges of switching signal RFSW, it is possible, with proper timing of the resulting pulse, to generate an index area specifying signal SI shown in FIG. 6E.
In the case of the 8 mm video tape recorder, considering the jitter of the VTR driving system, the non-uniformity of the mechanical arrangements of the respective VTRs and the influence of the stretching and shrinking of the magnetic tape, it is possible that data formed in the tracks on the tape may be displaced as much as .+-.1.5H from the expected positions. Such tolerable error of .+-.1.5H, is generally accommodated by a rotary head type recording and/or reproducing apparatus.
However, if such tolerable timing error arises because of the above mentioned influences and, after the PCM data and the video signal are recorded, the index signal is recorded in the index area of the recording track or erased therefrom in the after-recording mode, the index area specifying signal generated from the switching signal RFSW, as described above, may result in the index area being displaced by an amount corresponding to the timing error relative to the recording track. Thus, the index signal may be recorded in and/or erased from a part of the PCM audio data area AP or the video signal area AV with the result that a portion of the PCM audio data or the video signal is erased.